All of the solar cells currently used in households and large-scale power generation facilities are inorganic solar cells using silicon, CuInSe or other kinds of inorganic compound semiconductors. The efficiency of photoelectric conversion from sunlight energy into electric energy by such solar cells is within a range from 10 to 20% (for example, see Non-Patent Document 1). However, inorganic solar cells are expensive due to the costly process of silicon crystallization and film formation. To recover initial investments, it is necessary to further improve their photoelectric conversion efficiency and lower their production cost.
Meanwhile, organic solar cells, such as organic thin-film solar cells or dye-sensitized solar cells, which use organic substances as active layers or charge transport materials, are expected to be the next generation of solar cells since they are inexpensive and they can be used for realizing lightweight, flexible power generation devices. Accordingly, a number of worldwide industrial organizations, research institutes and universities have been conducting research on organic solar cells from various perspectives, such as the development of new materials for organic solar cells, the improvement of the production process, the optimization of the device structure, the enlargement of the device area, and the application of a high-productivity process using the Roll-to-Roll system.
In particular, an organic thin-film solar cell called the Bulk Heterojunction (BHJ) type is ranked as one of the most promising next-generation solar cells since it has the potential of being produced at even lower costs due to the simple production process and the availability of various organic materials (for example, see Non-Patent Document 2). The BHJ type organic thin-film solar cell has a power generation layer composed of a mixture of a high-molecular donor material and an acceptor material, where the high-molecular donor material and the acceptor material are phase-separated from each other on a nanoscale, allowing electric charges to be easily separated. Well-known examples of the acceptor material are fullerene derivatives (typically, [6,6]-Phenyl-C61-Butyric Acid Methyl Ester, or PCBM).
FIG. 7 is a schematic diagram for explaining the principle of photoelectric conversion by a BHJ type organic thin-film solar cell 100. The BHJ type organic thin-film solar cell 100 is formed by mixing a high-molecular donor 101 with an acceptor 102 made of a fullerene derivative (typically, PCBM). When a ray of light impinges on the donor 101, an exciton consisting of a positive hole and an electron combined as one pair is generated. The exciton diffuses to the junction plane, where the pair is dissociated into free carriers. The generated electron moves into the acceptor 102, to be extracted through an electrode 104, while the positive hole is extracted through an electrode 103 of the donor 101.